1. Field of the Invention
This disclosure is related to the field of window films having low emissivity (low e) for minimizing transfer of thermal energy by radiation, and that are suitable in electromagnetic interference (EMI) shielding applications.
2. Description of Related Art
Glass windows with a low emissivity (low e) are designed to allow frequencies in a specific bandwidth to pass through the window, such as visible light, while reflecting other frequencies outside of this desired bandwidth, such as infrared (IR). The low emissivity creates a high reflection of certain waves in the IR spectrum and serves to improve the thermal insulation of windows in buildings and vehicles. Thus, these low e windows find particular use in cold climates for preserving the heat in homes, offices, and automobiles and other heated environments, mitigating the escape of the warm interior air to the cold exterior via the window. These low e windows are also useful in hot climates for rejecting thermal energy radiation from the heated exterior via the window and thereby maintaining the cooler temperature of the interior. These windows are effective in providing comfort, visibility, and increased energy efficiency.
Window glass itself may be manufactured to provide the low e characteristics. During the manufacturing process and prior to installation, the glass is treated and/or coated with thin metallic layers, among others, to achieve the desired IR reflection. Examples of such treating and coating are described in U.S. Pat. Nos. 6,852,419 and 7,659,002. This treated glass, however, is problematic for several reasons. Firstly, it generally does not provide adequate protection against ultraviolet (UV) radiation. Secondly, the metal or other coating may not be sufficiently protected against the environment, resulting in a decreased mechanical strength and subjecting the glass to corrosion. In this regard, if or when the glass corrodes or breaks, the entire window must be replaced. Not only is this costly, but it can also be difficult to match the appearance and color of the original surrounding glass windows.
A more practical and economically efficient approach has been to utilize flexible polymeric films that can be adhered to the window glass. Such films are in widespread use and provide a variety of solar control functions. The films are easy to apply, can conveniently be removed and replaced, and can readily be made to duplicate the color and appearance of the film that is being replaced. Also, flexible films facilitate retrofitting of existing clear glass window panes and can impart solar control functions to the same. In this regard, the polymeric films also supply a level of protection from UV damage to household items, for example, fading of furniture.
The majority of solar control films are made by metalizing a polymeric substrate film, usually poly(ethylene terephthalate) (PET), and then laminating a second film of PET onto the metalized surface of the substrate film. These prior solar control films, however, sacrifice visible light transmittance (i.e., the amount of visible light that passes through the film, “VLT”) to achieve the desired emissivity, or vice versa, and have been limited to emissivities on the order of about 0.3, at best.
An example of such low e window film (on the order of about 0.3) is disclosed in U.S. Pat. No. 6,030,671. This and other previous low e window films utilize a metallic layer to reflect the IR radiation; however, metal is susceptible to corrosion, scratching, and abrasion. Thus, in such an application, a protective hardcoat is placed over the metal layers and facing the interior space to be reflected (i.e., the interior of the room). This protective hardcoat is a conventional cross-linked acrylate polyester based coating, and is necessary to supply the film with resistance to cracking, corrosion, scratching, and abrasion.
Since this hardcoat is IR absorbing and located between the IR reflective metallic layer and the interior of the room, it decreases the composite emissivity of the film. Thus, the hardcoat thickness resulted in a compromise between being sufficiently thick to function as a protective coat, whilst keeping IR absorption to a minimum. In any event, the hardcoat generally did not provide sufficient abrasion resistance, and when the hardcoat was thick enough to supply the film with the necessary durability, there remained serious detrimental effects on the emittance values. For example, U.S. Pat. No. 6,030,671 describes the thickness of hard coat placed over the optical layers (i.e., the PET and metallic layers) as being between 1-3 microns, and a hardcoat having a thickness of 3.0 microns will result in film composite having an emissivity of greater than 0.35. Additionally, in order to achieve this composite emissivity of the film, the visible light transmittance (VLT) was limited to about 50%.
In addition to managing IR radiation, there exists a need to control electromagnetic radiation. Electromagnetic radiation of various frequencies is radiated from many devices used in a wide range of facilities including homes, workplaces such as offices, manufacturing and military installations, ships, aircraft and other structures. Examples of such devices include computers, computer monitors, computer keyboards, radio equipment, communication devices, etc. If this radiation escapes from the facility, it can be intercepted and analyzed for the purpose of deciphering data associated with or encoded in the escaped radiation. For example, technology exists for reconstructing the image appearing on a computer monitor in a building from a remote location outside the building or from a location within a building by detecting certain wavelength frequencies from the monitor screen even if the monitor screen is not in view from the remote location. This is accomplished by known techniques wherein certain frequencies of light from the monitor screen, even after being reflected from various surfaces inside the building or room where the monitor is located, escape and are intercepted and analyzed by an eavesdropper in another location outside the building or room where the monitor is located. Obviously, the ability of an eavesdropper to intercept such radiation constitutes a significant security risk, which is desirably eliminated from facilities where secrecy is essential.
Although walls, such as brick, masonry block or stone walls may effectively prevent the escape of light frequencies from a facility, radio frequencies pass through walls that are not properly shielded to prevent such passage. Moreover, windows allow the passage of radiation to the outside where it can be intercepted, and can permit entry of various forms of radiation, such as laser beams, infrared, and radio frequencies, into the facility. As a result, sensitive or secret data may be gathered from within the structure.
Indeed, the United States Government has long been concerned by the fact that electronic equipment, such as computers, printers, and electronic typewriters, give off electronic emanations. The TEMPEST (an acronym for Transient Electromagnetic Pulse Emanation Standard) program was created to introduce standards that would reduce the chances of leakage of emanations from devices used to process, transmit, or store sensitive information. This is typically done by either designing the electronic equipment to reduce or eliminate transient emanations, or by shielding the equipment (or sometimes a room or entire building) with copper or other conductive materials. Both alternatives can be extremely expensive.
The elimination of windows from a structure would obviously minimize the above-noted security risk. The disadvantages of a windowless or enclosed structure, however, are self-evident. It would be highly desirable, therefore, to prevent the escape of radiation associated with data through windows while allowing other radiation to pass through so that the enjoyment of the visual effects provided by the windows can be obtained without an undue security risk.
The need for reducing the undesirable effects of electromagnetic radiation has led to the development of window filters and films to block the transmission of unwanted electromagnetic interference (EMI). These EMI shielding films, however, generally do not have the desirable low e and high VLT discussed above.
Given both the endless need to improve energy efficiency and the importance of security in today's competitive marketplace, a film that could preserve both energy and electronic privacy while maintaining adequate protection from the exposed environment is needed.